1. Field of the Invention
The present invention generally relates to a communication system and a communication method suitably used for a cellular wireless communication system and a communication apparatus used by a base station of this communication system. More particularly, this invention relates to a communication system, a communication method and a communication apparatus suitable for use in a system which transmits a CDMA (code division multiple access) signal by radio, for example.
2. Description of the Related Art
A proposed digital cellular wireless communication system for effecting a data communication between a base station and a plurality of terminal stations transmits down-link data from a base station to terminal stations with a frame arrangement shown in FIGS. 1A and 1B. This digital cellular wireless communication system shows an example of a transmission arrangement of a system called an HDR (high data rate). FIG. 1A is a diagram showing a frame arrangement in which slots that are divided at every unit length are located continuously. FIG. 1B shows an arrangement of one slot in which there are disposed a first data interval, a first pilot interval, a second data interval, a third data interval, a first power control information interval, a second pilot interval, a second power control information interval and a fourth data interval, in that order. In FIG. 1B, each power control information interval is shown as an “RPC” for simplicity.
As shown in FIG. 1B, a preamble signal is disposed at a part of the starting portion of the first data interval. Data within the preamble signal indicates a terminal station to which the data within the slot is transmitted. The first to fourth data intervals are set to be the same duration. To be concrete, the first to fourth data intervals are respectively set to be 464 chips, the first and second pilot intervals are respectively set to be 96 chips, the first and second power control information intervals are set to be 64 chips, respectively, and the length of one slot is set to be 2176 chips, for example.
In the case of this digital cellular wireless communication system, a data interval within one slot is allocated as an interval for transmitting data to one terminal station. Accordingly, when data should be transmitted from the base station to a plurality of specific terminal stations, a plurality of terminal stations, for example, transmit data by sequentially using one slot each. The power control information intervals (RPC) located ahead of and behind the second pilot interval individually transmit power control information to respective terminal stations which are communicating with the base station. This power control information is data for designating a transmission electric power of an up-link through which data is transmitted from each terminal station to the base station and instructs individual terminal stations either to increase a transmission electric power or to decrease a transmission electric power.
In order to simultaneously and individually transmit the power control information to individual terminal stations, in the proposed system according to the related art, power control information for each terminal station is spread by using a code allocated at every terminal station and the power control information thus spread for each terminal station are multiplexed and then transmitted. The processing in which a plurality of data thus spread by using the code corresponds to a CDMA (code division multiple access) system processing.
FIG. 1C is a diagram showing the manner in which power control information is transmitted in the first and second power control information intervals. In this example, power control information is simultaneously transmitted to six terminal stations of users #0 to #5, and power control information for six terminal stations are held at the same transmission electric power, spread, multiplexed and then transmitted.
Transmission electric powers of all signals transmitted from the base station during each slot period are set to the same value (fixed value), and the total transmission electric power of the power control information transmitted during the first and second power control information intervals also is set to be this fixed value. Accordingly, as shown in FIG. 1C, for example, when the power control information is simultaneously transmitted to the six terminal stations, a transmission electric power of each power control information becomes ⅙ of the fixed value. While the power control information to the six users #0 to #5 are illustrated in the state that they are simply added for simplicity in the example shown in FIG. 1C, in actual practice, the power control information that had been spread by using the code is added.
In the case of this digital cellular wireless communication system, a modulation system and a coding ratio of data transmitted from the base station during the data interval are adaptively set in response to the communication state between the base station and the terminal station of the called party so that even the arrangement in which the transmission electric power is set to the fixed value as described above can cope with the change of the wireless communication state between the base station and the terminal station.
FIG. 2 (formed of FIGS. 2A and 2B drawn on two sheets of drawings so as to permit a use of a suitably large-scale) is a block diagram showing an example of an arrangement of a transmission system of a base station for transmitting data to each terminal station with the arrangement of the frame shown in FIGS. 1A and 1B, for example. As shown in FIG. 2, there is provided a transmission data generating section 10 which is a circuit for generating data transmitted to a terminal station. A reception power management section 11 is a circuit for generating power control information transmitted to the terminal stations. Power control information to individual terminal stations are supplied to individual terminals (these terminals will hereinafter be referred to as “power control information input terminals”) 12a to 12n. A pilot channel input terminal 13 is a terminal to which pilot data is supplied from a pilot data setting circuit (not shown).
Transmission data generated from the transmission data generating section 10 is supplied to a data transmission processing section 14, in which it is processed for transmission such as transmission coding processing, modulation processing and interleave processing. The data processed herein is data located in the first to fourth data intervals of the slot arrangement shown in FIGS. 1A and 1B, and the preamble data located at the starting portion of the first data interval also is processed. Since data located in the first to fourth data intervals are fundamentally located to one terminal station at the unit of slots as earlier noted, during a period in which data of one slot is processed, the coding system and the modulation system suited for the terminal station for effecting a communication in that slot are set by the data transmission processing section 14 and the transmission processing is executed.
The coding system and the modulation system suitable for the terminal station are set based on bit rate information supplied from a data rate control section 15 to the data transmission processing section 14. To be concrete, when data which is to be transmitted to a terminal station of which communication state (receiving situation at the terminal station) is satisfactory is transmitted, transmission data is coded by a multivalued modulation such as a 16QAM (16-quadrature amplitude modulation) and high coding rate and then transmitted at a high throughput. When transmission data which is to be transmitted to a terminal station whose communication state is not satisfactory is transmitted, transmission data is coded by a low coding rate and a QPSK (quadrature phase shift keying)-modulated signal is spread or transmitted a plurality of times and thereby transmitted at a low throughput. Transmission data (I-channel data and Q-channel data) that had been processed by the data transmission processing section 14 are supplied to a channel multiplex circuit 16.
Power control information supplied to the power control information input terminals 12a to 12n are information prepared individually at a plurality of terminal stations with which the base station communicate at the same period, and individual information are separately supplied to the power control information input terminals 12a to 12n at every terminal station. Accordingly, power control information are generated and supplied in response to the number of terminal stations which communicate with the base station at the same period. Power control information supplied to one terminal station is 1-bit information per slot. Such 1-bit information is used to instruct the corresponding terminal station to increase a transmission electric power or to decrease a transmission electric power.
Respective power control information applied to the power control information input terminals 12a to 12n are supplied to direct sequence sections 17a to 17n, in which each 1-bit data is broken into four-times data, i.e., 4-bit data in which 1-bit data is repeated four times. The 4-bit power control information per slot are respectively supplied to respective spread processing circuits 18a to 18n, in which they are spread and modulated into data having a predetermined-time bit rate (bit rate of 32 times) by using a predetermined code (e.g., Walsh Code) set at every terminal station and thereby I-channel power control information and Q-channel power control information are obtained. Since the 4-bit data is spread by 32 times, there may be obtained data having a data rate of 128 chips/slot.
The data spread and modulated by the respective spread processing circuits 18a to 18n are supplied to a symbol multiplex circuit 19, in which they are mixed as signals of one system. The signals thus mixed are supplied to a variable gain setting circuit 20. The variable gain setting circuit 20 adjusts the gains of the multiplexed power control information such that the gains may become constant values. Specifically, in the case of this example, the gain of the signal transmitted from the base station is a previously-determined constant value. The number of data which are multiplexed by the symbol multiplex circuit 19 is changed in response to the number of terminal stations which are communicating with the base station at that time. Therefore, in response to the number of data multiplexed by the symbol multiplex circuit 19, the variable gain setting circuit 20 generates a signal having a constant gain by adjusting the gain of the transmission signal. The signals whose gain had been adjusted by the variable gain setting circuit 20 are supplied to a channel multiplex circuit 16.
Pilot data applied to the pilot channel input terminal 13 are all null data and hence supplied to the channel multiplex circuit 16 as they are.
The channel multiplex circuit 16 multiplexes the respective signals supplied thereto in a time-division manner such that the slot arrangement shown in FIG. 1B may be obtained. The signals thus multiplexed in a time-division manner by the channel multiplex circuit 16 are supplied to a scramble processing circuit 21. The scramble processing circuit 21 is supplied with spread codes of both I-channel and Q-channel from terminals 22i, 22q and spreads data by using these spread codes. The spread transmission signals of I-channel and Q-channel are supplied to a digital-to-analog (D/A) converter 23, in which they are converted from digital data into analog data. The analog transmission signal is supplied to a high-frequency (radio-frequency) circuit 24, in which it is processed as a high-frequency signal and thereby converted into a signal having a predetermined transmission frequency channel. Then, the signal having the converted transmission frequency is supplied to an antenna 25, from which it is transmitted to each terminal station by radio.
Since the transmission signal is transmitted from the base station to each terminal station as described above, the base station is able to individually communicate with respective terminal stations at the unit of slots and is able to simultaneously transmit the power control information for instructing the transmission state of each terminal station to all terminal stations at every slot. With respect to the wireless transmission of down-link information from each terminal station to the base station, the transmission electric power can properly be set based on the power control information transmitted from the base station and the base station can satisfactorily receive the signal from each terminal station. With respect to the wireless transmission of down-link information from the base station to each terminal station, since the coding ratio and the modulation system are set adaptively although the transmission power is fixed to the constant value, the transmission signal can be satisfactorily received by any terminal station. Specifically, if each terminal station is a mobile station, then although a distance or a communication state between each terminal station and the base station is changed at any time, the above processing is effected on the transmission signals of up-link information and down-link information respectively, whereby the transmission processing following the change of the distance or the communication state is set adaptively so that wireless communication can constantly be carried out satisfactorily.
Since the power control information multiplexed and simultaneously transmitted are spread by individual codes at every terminal station and then transmitted, each terminal station can receive only its own power control information by despreading the received signal with the code allocated thereto, and hence each terminal station can receive power control information properly.
In the above transmission processing, although the power control information transmitted from the base station to the respective terminal stations are multiplexed and then simultaneously transmitted to all terminal stations, the transmission processing of the power control information is constantly the same processing so that all terminal stations which communicate with the base station cannot always receive power control information satisfactorily. Specifically, with respect to data transmitted during the data interval of each slot period, one slot is allocated to one terminal station so that the coding ratio and the modulation system suitable for such terminal station can be set. However, since power control information should be transmitted to respective terminal stations simultaneously, it is difficult to change the signal processing at every information transmitted to individual terminal stations.
Accordingly, while a terminal station located close to the base station, for example, can satisfactorily receive the power control information, it is frequently observed that a terminal station existing at the peripheral portion of a service area comprised of base stations is poor in reception situation of power control information. In particular, if one base station communicates with a large number of terminal stations at the same time, then a lot of power control information should be multiplexed and transmission electric powers of individual power control information are lowered unavoidably. As a consequence, the situation under which the terminal station existing at the peripheral portion of the service area receives power control information is degraded considerably.